Generally, digital control systems have a variety of inputs to a system processor with differing frequencies. These inputs may be affected by system noise such as line frequency noise, switching noise or crosstalk between various input signal lines in the system. If the input signals have a low frequency relative to the system noise, the noise can be filtered by sampling the input so as to satisfy the Nyquist criterion of the system noise, and applying these samples to a digital filter.
However, in some systems, the extra processing required due to sampling at the higher rate cannot be accommodated by the system processor. For example, in a time multiplexed system, a processor sequentially processes channel input samples from a number of channels. The maximum processing time allotted to a given channel depends upon the sampling rate at which each channel is sampled. Thus, sampling at a rate to satisfy the Nyquist criterion of the system noise restricts the amount of time available to process a channel input sample. The processor must execute the digital filter algorithm and the necessary system signal processing within the time allotted for the channel. The system signal processing can include, for example, limited checking and linearizing the sample channel inputs.